The present invention relates to photographic printing systems. In particular, the present invention is an improved method and apparatus for use in a photographic printer which compensates exposure times based upon the gamma values of the photosensitive medium upon which prints are being made.
Photographic printers produce color or black and white prints or transparencies from photographic film originals (generally negatives). High intensity light is passed through the film and imaged on the photosensitive print medium (film or paper). The photographic emulsion layers on the print paper or film are exposed and subsequently processed to produce a print or transparency of the scene contained in the original.
A critical portion of a photographic printer is the exposure control, which controls the exposure of the photosensitive medium to ensure that the image on the photosensitive member is properly exposed. The exposure control may utilize inputs from several different sources in order to determine the proper exposure. For example, most automatic printers use large area transmission density (LATD) sensors to sample light transmitted by the negative either prior to or during the exposure. Control of the exposure is determined using a method known as "integration to grey". In addition, many automatic printers include an automatic density correction (ADC) or color/density scanning station whih scans the negative prior to printing and corrects the exposure in the event of a condition known as "subject failure". Finally, the operator may enter density correction signals from the operator control panel. Based upon some or all of these input signals, the exposure control determines the proper exposure for each of the color channels or for one black and white channel.
One characteristic of the photosensitive medium which affects exposure is the "gamma" of each photosensitive emulsion layer. Gamma denotes the slope of the linear portion of the "D log E curve", which is a plot of print density as a function of the log.sub.10 exposure. If gamma is 1, doubling the exposure increases the resulting density of the print by 0.3 density points. If gamma is greater than 1, doubling the exposure increases density by more than 0.3 density points. Conversely, if gamma is less than 1, doubling the exposure increases density by less than 0.3 density points. In other words, indentical changes in exposures of photosensitive media having different gamma values will result in different changes in densities.
Photofinishers generally attempt to buy photographic paper in large quantities so that the emulsions for the entire batch of paper are the same and, therefore, the gamma values do not change. When it becomes necessary, however, to change paper from one batch to another, the gamma values may be different. As a result, the density change resulting from the same exposure change may differ from one batch of paper to another.
The effects of variations in gamma can be significant in the operator-entered button corrections for overall density or for density for individual colors. In the prior art printers, a +1 button correction may vary anywhere from about 0.05 density points to about 0.15 density points. Some prior art printers permitted adjustment of the button increments by changing resistance values to pick the desired density increment for the buttons corresponding to an average gamma value. As the gamma values vary from this average value, of course, the density increments of the button corrections change.